1. Field of the Invention
The invention relates to a kind of programmed isolating starting system and method of a switching power supply. It is applied to the power supply made by a pulse width modulator.
2. Related Art
A switching power supply is one kind of power supply, which is mainly applied to a pulse width modulator (abbreviated below as PWM), and is used to modulate the output voltage. However, because of the demands of safety regulations and the nature of electricity, it is necessary to isolate the electricity between the primary circuit and the secondary circuit. There are two kinds of isolating methods at present:
A. Put the PWM at the primary circuit; the secondary feedback signal is isolated by the xe2x80x9coptocouplerxe2x80x9d or the xe2x80x9cisolated feedback generatorxe2x80x9d. This signal is sent to the controlling IC (integrated circuit) of the primary PWM to modulate the pulse width, and to drive the starting switch. Another kind of magnetic amplifier circuit controls the magnetic reset current via the secondary feedback signal, so it can suppress the valid magnitude of the secondary pulse width to achieve the objective of regulating the power.
B. Put the PWM at the secondary circuit. The primary circuit is used as an isolating auxiliary power supply, which supplies the necessary power to the controlling IC of the PWM. The output (secondary) feedback signal is sent directly to the PWM to control the IC, and couples and drives the primary switch via the isolation of the driving transformer.
In the isolating method A, the starting method is considered to save power. Therefore, the starting current is provided by the input power to drive the switch in an on-off state of the power transformer to couple the energy. One additional winding of auxiliary power is wound on the power transformer in order to supply the power when the controlling IC of the PWM works continuously. The power saved=(input voltagexe2x88x92auxiliary power voltage)*the working current of PWM IC. The starting current of this starting method is much smaller than the working current; the working voltage (auxiliary power) is about 10-20 Volts.
In the isolating method B, in general, the isolating auxiliary power is directly used as the starting power.
The isolating method via the optocoupler in method A above is quite easy for the circuit. However, it has several restrictions: 1. The error of the current transfer ratio (CTR) is very large when products are made. 2. The magnitude of the driving current and the change of temperature can have a large influence on the CTR, with a non-linear relation. 3. The stability is not good, and the CTR decreases with time. 4. The bandwidth is not wide and the echo response is not good. 5. Compensation needs to be done twice.
The circuit isolated feedback generator in method A is quite complicated. Although a specific IC (UC3901) can simplify the circuit, there are still several problems and restrictions as follows: 1. The frequency of the isolated feedback generator (about 1-5 MHz) is different from the power switch. Since the layout is not good and interference is common, it is not easy to compensate. 2. It is not popular and the market is not big enough. 3. The cost is quite high. 4. Compensation needs to be done twice.
The circuit of the isolating method with the magnetic amplifier in method A is complicated. Although the specific IC (UC3838) can simplify the circuit, there are still several problems and restrictions as below: 1. Very large reset current will be necessary during the short circuit protection. The saturated core (which is equal to a switch) becomes very hot due to the rapid increase of core loss, so the character of the core material changes. Unless heat radiating measures are appropriate (e.g., enlarging the iron core), the short circuit cannot be too long. 2. The saturated core should be added to increase the volume. 3. The cost is quite high. 4. Compensation should be done twice.
There are two methods using B as auxiliary power:
I. The AC input can use a low frequency transformer as auxiliary power during isolation. However, there are some problems and restrictions as below: 1. The volume of the low frequency transformer is large and it occupies more space. 2. The efficiency is low, and the power loss is high. 3. A voltage regulator should be added for the secondary circuit, so the cost increases.
II. The DC input can use a feedforward feedback PWM circuit or a ringing chock converter (RCC) circuit for auxiliary power isolation. However, there are some problems and restrictions as below: 1. The effect of the power regulation is not good, and is easily influenced by temperature. 2. The efficiency is low, and the power loss is great. 3. The frequency between the auxiliary power and the main power is different and they easily interfere with each other. 4. The circuit is too complicated, so the cost is high.
The problems for the starting method are: In method A, it is started by the input power directly. An additional device is necessary for primary/secondary feedback isolation. In method B, the voltage of the auxiliary power changes as the main output load varies. The voltage may exceed reasonable working voltage values.
In addition to the above isolating method, the switching power supply needs to have protective measures for excessive current and short circuit, for its own safety and the safety of its users. The way of doing this is to induce the current to the voltage and input this induced voltage value into the current limit comparator. This value is compared with a reference potential. When a short circuit happens, it causes the pulse width in the PWM control IC to close. Hence, the current works Pulse-by-pulse. This periodical frequency is usually much lower than the work frequency, and the pulse width is very small.
However, if the short circuit protection is too long, it is easy to cause damage to the components. The periodical pulse causes the current limit. Although it has the foldback current limit manner, the output is limited under small root mean square (abbreviated as rms below) current value. The momentary current is even larger than the maximum output current (sometimes much larger). When the switch works continuously and the output circuit is short, the total energy loss=(Vin rms)xc3x97(Iin rms), the energy loss on the switching power supply=(Vin rms)xc3x97(Iin rms)xe2x88x92(Vout rms)xc3x97(Iout rms). Since the Vout rms is very small, the power module generates a great deal of heat. As the heat accumulates, the temperature of the power module increases, and the safe operating area of the components decreases. When it reaches the critical limit, it causes damage very easily.
According to the above problems in the prior isolating and starting methods of the switching power supply, the invention provides a kind of programmed isolating and starting method and device of the switching power supply. The circuit is easy and its volume is small. Because it is a software application, it omits the use of hardware.
Another objective of the invention is to provide a kind of programmed isolating starting method and device of the switching power supply for which feedback compensation is easy. The output voltage directly sends feedback to the error amplifier of the IC. The square-wave driving signal passes through the transformer T2, and then is isolated to the primary driving switch (transistor). This is regarded as a liner relation in the mathematics mode. Because its bandwidth (the bandwidth of the error amplifier) is larger, it is easy to design the compensator.
Another objective of the invention is to provide a kind of programmed isolating and starting system of the switching power supply, whose starting circuit has an isolating function. The circuit has an automatic protection function for the over-current/short circuit, and a judging and protecting function of the over-current/short circuit at the temporary state or long time state.
Another objective of the invention is to provide a kind of programmed isolating starting system of the switching power supply, for which the time for starting power is very short. The power loss is low, and there is no problem of loss of the auxiliary power conversion for a long time.
According to the programmed isolating starting system of the switching power supply provided in the invention, the PWM is put at the secondary circuit. It is not used as an isolating auxiliary power. Instead, it is used as an isolating starting power. The secondary power output circuit provides the necessary power for the PWM to control the IC. The starting power uses a device that contains a program to control the starting, e.g. a Micro-control Unit. Once the starting is finished and the PWM controlling circuit works normally, the program stops working. If the system needs to restart, it can automatically restart via the hardware and the program of the invention. Even when the input voltage has a very large difference, the system can be started and the working voltage can be provided via the hardware and the software.
The programmed isolating and starting method of the switching power supply in the invention has the advantage of being a simple circuit, with little noise, small volume, lower cost and easy compensation, and is appropriate for the military market. Another benefit is to have an automatic or intelligent over-current/short circuit protection function. When the output is over-current or short circuited, there is almost no energy loss, and the additional circuit can be omitted or can be very simple.
Further scope of applicability of the invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.